Users with more than 2,000 reputation can edit posts as they want, without any control, right?
So, if a reputed user makes an invalid edit, is there a way (i.e., a flag) to inform that that edit is invalid? Or one has to re-edit the post?
It depends a bit on the scope of the mistake that was made; if a botched edit is obvious and easily fixed then there's nothing more that really needs to happen. Leaving a comment to the effect of 'I fixed your edit' really doesn't add anything of value to the post. It's fixed - move on :)
If it's a controversial edit that requires a protracted discussion to settle, we ask that you use chat for it whenever possible. You can
If just a few comments are needed, then go ahead and just have a short chat in comments, just remember to flag them as obsolete (or delete them yourselves) once the edit is settled. As Yannis notes in comments, just mention them, they'll be notified.
If it's really bad, just consider rolling back the edit entirely, then perhaps editing the post yourself if there's something actually wrong with it that simply wasn't fixed by the previous edit attempt.
Above all, just try to keep discussion in a friendly and collaborative tone - differences in opinion when it comes to editing can sometimes get really heated, and that's not doing the author of the post or anyone looking at it any good.
If you're both confident 1) that you are right and 2) that you can be definitively persuasive in about two sentences, manually fix the edit and leave your reason in the edit summary. Note this is poor practice if there's a chance it can lead to an edit war, but otherwise it's a pretty expedient way to get your point done.